U.S. patent number 4,373,774 [Application Number 06/142,473] was granted by the patent office on 1983-02-15 for illuminator for supplying a divergent illuminating beam from a predetermined area of a plane.
This patent grant is currently assigned to Thomson-CSF. Invention is credited to Michele Brevignon, Georges Dubroeucq, Michel Lacombat.
United States Patent |
4,373,774 |
Dubroeucq , et al. |
February 15, 1983 |
Illuminator for supplying a divergent illuminating beam from a
predetermined area of a plane
Abstract
An illuminator for supplying a divergent illuminating beam from
an area of predetermined energy distribution and dimensions
comprising a coherent source whose image is formed in a plane by
imaging means incorporating electrically controlled optical
scanning means making it possible to displace the image in the said
area in accordance with a predetermined law.
Inventors: |
Dubroeucq; Georges (Paris,
FR), Lacombat; Michel (Paris, FR),
Brevignon; Michele (Paris, FR) |
Assignee: |
Thomson-CSF (Paris,
FR)
|
Family
ID: |
9224602 |
Appl.
No.: |
06/142,473 |
Filed: |
April 21, 1980 |
Foreign Application Priority Data
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Apr 23, 1979 [FR] |
|
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79 10209 |
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Current U.S.
Class: |
359/202.1 |
Current CPC
Class: |
G03F
7/70058 (20130101); G03F 7/70075 (20130101); G02B
19/0014 (20130101); G02B 19/0047 (20130101); G03F
7/70583 (20130101) |
Current International
Class: |
G02B
19/00 (20060101); G03F 7/20 (20060101); G02B
027/17 (); G02B 027/48 () |
Field of
Search: |
;350/6.1-6.91,285
;250/230,235 ;346/109 ;358/206,208,285,293 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
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|
|
1259952 |
|
Mar 1961 |
|
FR |
|
2104599 |
|
Apr 1972 |
|
FR |
|
487318 |
|
Jun 1938 |
|
GB |
|
Other References
Anderson et al., R. J., App. Optics, vol. 10, No. 7, pp. 1605-1608,
(Jul. 1971). .
Bowman, M. J., "Two New Methods . . . ", Applied Optics, vol. 7,
No. 11, Nov. 1968, pp. 2280-2284..
|
Primary Examiner: Henry; Jon W.
Attorney, Agent or Firm: Oblon, Fisher, Spivak, McClelland
& Maier
Claims
What is claimed is:
1. A device for synthesizing a light source in a predetermined
plane, said light source having a regulatable energy distribution
and form and acting as an illuminator, said source supplying a
divergent illuminating beam from said plane having a predetermined
coherence level, comprising:
a coherent radiation source producing a coherent beam;
optical means for controlling the movement of said beam, including
electrically controlled optical scanning means for moving the beam
in a selected circular pattern so as to change the coherence level
and the amount of coherent noise in said beam; and
wherein said optical means includes two optical scanning mirrors
whose pivoting axes have different directions, two lenses
positioned between said two optical scanning mirrors, having a
common optical axis and a common focus, and a third lens for
focusing the beam from said second mirror to a point source which
moves according to the movement of said two scanning mirrors said
point source having a variable shape and a predetermined
distribution of energy levels.
2. A device for synthesizing a light source in a predetermined
plane, said light source having a regulatable energy distribution
and form and acting as an illuminator, said source supplying a
divergent illuminating beam from said plane having a predetermined
coherence level, comprising:
a coherent light source for providing a coherent beam;
a first lens;
a second lens;
wherein said first lens and second lens are coaxial with said
coherent beam, said first lens acting to focus said coherent beam
and said second lens acting on the focused beam from said first
lens to produce a collimated beam larger in diameter than said
coherent beam;
a first reflector for reflecting said collimated beam from said
second lens in a direction dependent on the orientation of said
first reflector;
a first deflecting device for changing the orientation of said
first reflector in accordance with a first deflecting control
signal;
a third lens for receiving the reflected beam from said first
reflector, focusing the same and being a distance from said first
reflector, equal to the focal length of said third lens:
a fourth lens for receiving the focused beam from said third lens
and producing a collimated beam therefrom which is larger in
diameter than said reflected beam and being a distance from said
third lens equal to the sum of the focal lengths of said third and
fourth lenses;
a second reflector for reflecting said collimated beam from said
fourth lens in a direction dependent upon the orientation of said
second reflector;
a second deflecting device for changing the orientation of said
second reflector in accordance with a second deflecting control
signal;
wherein said first and second reflectors rotate about first and
second axes, respectively, and which extend in perpendicular
directions;
a fifth lens for receiving said reflected beam from said second
reflector, focusing the same to form said light source and being a
distance from said second reflector equal to the focal length of
said fifth lens; and
wherein the movement of said first and second reflectors produces a
movement of said light source so that said light source has a
variable shape and a predetermined distribution of energy levels
and no coherent noise.
3. A device for synthesizing a light source in a predetermined
plane, said light source having a regulatable energy distribution
and form and acting as an illuminator, said source supplying a
divergent illuminating beam from said plane having a predetermined
coherence level, comprising:
a coherent radiation source producing a coherent beam;
optical means for controlling the movement of said beam, including
electrically controlled optical scanning means for moving the beam
in a selected ring shaped pattern so as to change the coherence
level and the amount of coherent noise in said beam; and
wherein said optical means includes two optical scanning mirrors
whose pivoting axes have different directions, two lenses
positioned between said two optical scanning mirrors, having a
common optical axis and a common focus, and a third lens for
focusing the beam from said second mirror to a point source which
moves according to the movement of said two scanning mirrors said
point source having a variable shape and a predetermined
distribution of energy levels.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an illuminator for supplying a
divergent illuminating beam from an area with predetermined energy
distribution and dimensions.
The manufacture of integrated circuits involves the superimposing
of levels in a semiconductor wafer (e.g. silicon or AsGa). For this
purpose, windows are formed on the wafer making it possible to
locate the implantation or processing by masking a photosensitive
resin deposited on the wafer from a mask. At present, the most
frequently used method is transfer by optical projection.
This projection can be carried out at scale 1, the mask being
completely projected onto the wafer. It can also be carried out by
splitting up the image, either by analysing the mask by a moving
slot, or by using the photorepetition method with a reduction of
scale.
In the latter case, during production it is necessary to optimise
the production flow rate, more particularly by reducing the
exposure time necessary for a good illumination. It is known to
bring about a significant reduction in this time by using a laser
source.
Due to its spatial and time coherence, the laser constitutes a
special illuminating source. Its use in an optical image projection
device leads to the appearance of a significant coherence noise.
The nature of the illumination must therefore be modified in order
to obtain an image with a good signal-to-noise ratio. For this
purpose, it is possible to use a device which can average out the
interference patterns produced by the coherence of the beam. It is
also possible to use a rotary diffuser located in the path of the
illuminating beam. The disadvantage of the diffuser is that it
introduces a very large luminous energy loss, which partly cancels
out the advantages resulting from the high available power of a
laser.
BRIEF SUMMARY OF THE INVENTION
The invention proposes a device offering a very good luminous
efficiency by creating a source whose degree of coherence can be
regulated at random. In more general terms, the invention aims at
synthesizing a source having regulatable energy distribution and
form, said source being constructed from a laser source and is
usable not only in a photorepeater, but in numerous image formation
devices where it is desirable to use the advantages of spatial
coherence, whilst eliminating the coherence noise.
The invention therefore relates to an illuminator for supplying a
divergent illuminating beam from a predetermined area of a plane,
wherein it comprises a radiation source and optical means making it
possible to form the image of the source in the said plane and in
which the said source is coherent and the said optical means
incorporate electrically controlled optical scanning means making
it possible to move the image in the area in question in accordance
with a predetermined law.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described in greater detail hereinafter relative
to non-limitative embodiments and the attached drawings, wherein
show:
FIG. 1 a diagram of an optical system for illuminating an object
according to the prior art.
FIG. 2 the intensity distribution of the image of a line by
different types of illumination.
FIG. 3 a device according to the invention.
FIG. 4 examples of synthesized sources.
FIG. 5 an explanatory diagram.
FIG. 6 the application of the invention to a photorepeater.
FIG. 7 a diagram showing the significance of the invention in
connection with the signal-to-noise ratio.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a diagram of an optical system for illuminating an object
from a source S, which supplies a luminous beam 100 of axis Z. An
object 4, which can be a photographic plate, a mask or any other
object formed from transparent areas and opaque areas is
illuminated by the beam. In front of the said object is placed a
condenser 3 which makes it possible to condense beam 100 towards
the entrance pupil 5 of an objective 9. The plane of pupil 5
coincides with the focusing plane of beam 100. The image of object
4 by objective 9 of exit pupil 6 is formed in a plane 7, which can
be materialised, as appropriate, by a screen or a photosensitive
layer such as a resin deposited on a semiconductor wafer which is
to form an integrated circuit. The image formed in plane 7 largely
depends on the characteristics of the source S, particularly its
spatial coherence or in other words its dimensions. An important
parameter, called the degree of partial coherence is the ratio
.sigma. between the diameter of image I of source S in the plane of
pupil 5 and the diameter of said pupil. When source S is a
perfectly coherent source, such as a laser, ratio .sigma. is
substantially zero. In this case, all the illuminated points of
object 4 are correlated in phase. In the image plane, the image is
reconstituted by interferences between the orders of diffraction
all transmitted without attenuation and order zero. When the
dimensions of source S increase, the correlation of phase
decreases. Certain orders of diffraction are partly blocked by
pupil 5, so that the image of the object is not precisely
reconstituted. The illumination is completely incoherent, when the
dimensions of the source are infinite. Thus, when the dimensions of
the source completely cover the pupil 5 (.sigma.=1), the
illumination is already substantially incoherent.
The influence of the source on the image obtained is demonstrated
by an example. With the exception of a transparent line, the object
is an opaque plate. FIG. 2 is a distribution diagram of the
luminous intensity in plane 7 in a direction X perpendicular to the
direction of the line for different sources. Curves C.sub.1,
C.sub.2, C.sub.3 correspond respectively to .sigma.=0.1,
.sigma.=0.5, .sigma.=1. The theoretical image of the line is shown
at C.sub.O. It is clear that the lower the degree of coherence the
higher the intensity collected at the centre of the line and the
steeper the slopes of the curve. To obtain image profiles which are
of good quality and resolution, it is preferable to have steep
slopes.
The advantages of a high spatial coherence on the resolution lead
to the use of either an incoherent source behind which is placed a
diaphragm, or a laser. The very low energy efficiency obtained with
a diaphragm source means that preference is given to the laser.
However, the very high time coherence of the latter creates a high
and disturbing coherence noise. This can be eliminated by
increasing the diameter of the source, which still remains low
compared with the diameter of the entrance pupil of the
objective.
FIG. 3 shows an illuminator according to the invention, which
serves to illuminate an object 4. This illuminator forms the image
S of a laser source 1, which is considered as a point source. The
parallel beam 300 supplied by laser 1 is widened by a system of
lenses 2 and is then reflected by a mirror M.sub.y rotating about
an axis .theta..sub.y towards a lens L.sub.1 of focal length
f.sub.1. The beam, which was parallel, thus converges in the focal
plane of lens l.sub.1. A second lens L.sub.2 of focal length
f.sub.2 is placed behind lens L.sub.1 in such a way that its object
focal plane coincides with the image focal plane of L.sub.1. Thus,
on leaving lens L.sub.2 there is once again a parallel beam which
is reflected by a second mirror M.sub.x which rotates about an axis
.theta..sub.x. The laser image is formed at S by a third lens
L.sub.3. When mirrors M.sub.x and M.sub.y are fixed, the
synthesized source is a point source and coincides with the focus
of lens L.sub.3. The directions of rotation axes .theta..sub.x and
.theta..sub.y are selected in such a way that the effect of the
rotation of mirrors M.sub.x and M.sub.y is a displacement of point
S in the image focal plane of lens L.sub.3 or in two perpendicular
directions. The focal lengths of lenses L.sub.1, L.sub.2 and
L.sub.3 are chosen as to obtain the desired apperture .gamma. for
the beam from source S. Mirrors M.sub.x and M.sub.y are for example
galvanometric mirrors controlled respectively by electrical signals
S.sub.x and S.sub.y. The latter can be fine-wave signals, sawtooth
signals or random signals. The rotation amplitudes given to the
mirrors determine the displacement amplitudes of point S. The
oscillation periods must be small compared with the observation of
exposure times provided. For a given time T the form and energy
distribution of the synthesized source is defined in the following
manner. During time T, point S describes a curve as a function of
signals S.sub.x and S.sub.y. This curve defines a surface .SIGMA.,
which gives the form of the source. Each point of the surface of
coordinates u,v has a certain brightness B.sub.o, but it is only
occupied for a certain time .sigma.t by point S. Its energy
distribution is written B(u,v)=B.sub.o .multidot.(.sigma.t/T). With
the knowledge of the curve described by point S and its time
derivative it is possible to calculate the function B(u,v). It is
possible to demonstrate that the source synthesized in this way by
scanning surface .SIGMA. by point S is equivalent to an incoherent
source (time incoherence) of the same form and same energy
distribution.
Examples of sources which can be synthesized are shown in FIG. 4.
At (a), there is a centered coherent source obtained when signals
S.sub.x and S.sub.y are zero. At (b) there is an offcentered
coherent source obtained when S.sub.x and S.sub.y are non-zero
constants u.sub.o, v.sub.o. At (c) there is a linear source of
length 2a along axis V, obtained with S.sub.x =0 and S.sub.y =a cos
.omega.t. In this case, the device need only have a single mirror.
At (d) there is a linear source forming an angle .alpha. with axis
u, obtained with S.sub.x =u.sub.o cos .omega.t and S.sub.y =v.sub.o
cos .omega.t. At (e) there is a rectangular source of size 2a and
2b obtained by a television-type scanning with S.sub.y =b sin
.omega..sub.y t and for S.sub.x a periodic signal of high pulsation
.omega..sub.x compared with .omega..sub.y, for example a sine curve
of amplitude a or a sawtooth signal. At (f) there is an annular
source of radius R.sub.o, obtained with S.sub.x =R.sub.o cos
.omega.t, and S.sub.y =R.sub.o sin .omega.t. At (g), there is an
elliptical source obtained with S.sub.x =u.sub.o cos .omega.t and
S.sub.y =v.sub.o sin .omega.t.
As apparent from FIG. 3, the mirrors M.sub.x and M.sub.y are not
arranged in a random manner in the optical path. To prevent
aberrations and light losses level with condenser 3, it is
preferable that there are not only small displacements of point S
relative to the diameter of the condenser, but also that the axis
of the following beam on the condenser is always parallel to its
optical axis. Thus, the divergent light beam which reaches the
condenser is always substantially centered with respect to the
latter and there are no light losses. In order to obtain this
parallelism, according to the invention, each mirror is positioned
in such a way that its rotation axis passes through the focus of
the lens which follows it. The optimum case is encountered when the
rotation axis is entirely contained within the focal plane, as
occurs with mirror M.sub.y. Thus, when this mirror pivots, point I
describes a straight line in the plane of pupil 5.
FIG. 5 shows the path of incident beam 300 and the beam reflected
on mirror M.sub.y. At (a) mirror M.sub.y is in a mean position and
the axis z of the beam converges in the focal plane PF at A, which
is the focus of the lens. At (b), as the mirror has rotated by an
angle .theta./2 compared with the previous position about an axis
perpendicular to the plane of the drawing and passing through the
object focus O of lens L.sub.1, the axis of the reflected beam
forms an angle .theta. with the optical axis of lens L.sub.1. The
beam is still focused in plane PF at point B. A simple optical
construction shows that the axis z.sub.1 of the beam on the lens is
parallel to optical axis z and is spaced from the latter by a value
equal to f.sub.1 .times.tg.theta.. The same reasoning can be used
for mirror M.sub.x in a perpendicular direction.
In the case of the device shown in FIG. 3, for constructional
reasons, this position of the rotation axis has not been respected
for mirror M.sub.x. When this mirror pivots, point I describes a
parabolic axis in the plane of the pupil. However, as the rotation
amplitudes still remain low (typically below 0.1 rad) the maximum
variations obtained compared with the ideal straight line are very
small (relative variation below 4%). Furthermore, the axis of the
light beam only remains parallel to the optical axis of the lenses
when it passes through the foci thereof, which is the case in FIG.
3. If this is not the case, the axis of the beam remains parallel
to a fixed direction which is not parallel to the optical axis.
The invention is applied more particularly to making a pattern
carried by a mask on a photosensitive resin by means of a
photorepeater. FIG. 6 shows a photorepeater using a device 200 for
synthesizing a source S in accordance with the invention. Laser 1
is for example a krypton laser of power 2 W, whose radiation
diagram has two very fine lines at 413 and 415 nm. In the path of
beam 300, there are successively a fast shutter 14 which is used
for determining the necessary illumination time T, then the various
elements of FIG. 3 indicated in the same way, namely lenses 2,
system 200 constituted by mirrors M.sub.y and M.sub.x and lenses
L.sub.1, L.sub.2 and L.sub.3. This system supplies the source S
providing the divergent beam 100 illuminating the entire surface of
condenser 3. Thus, lens L.sub.3 has a large aperture and a small
focal length. Behind condenser 3 are provided mask 4 carried by an
object mount 8, objective 9 carried by an objective holder 10 and
which is provided with sensors 11 ensuring automatic focusing. A
semiconductor wafer 12 covered with a photoresist material is
placed in the image plane. It is carried by a table 13 which
ensures displacement in two perpendicular directions for the
repetition of the pattern carried by the mask. The main noise
sources in this type of device are:
in the case of noise due to the object the phase noise due to the
lack of uniformity in the thickness and possibly the index of
refraction of the mask and the impulse noise generating by mask
faults, this constituting white noise which is not dependent on the
coherence;
in the case of noise due to the projecting objective, the random
noise due to diffusion in the glass of the lenses.
In the case of coherent lighting, as the source is substantially a
point source and the time coherence is very high, the phase noise
and diffusion noise are summated at any point in the image plane in
amplitude and in phase. When the scanning according to the
invention is used, the diffusion noise is reduced by enlarging the
image of the source in the entrance pupil of the objective. The
phase noise is reduced by varying the phase of the wave
illuminating the mask during the exposure time. In the case of a
circular or annular source, as shown at (f) in FIG. 4, the
theoretical value of the surface of image I of the source in the
entrance pupil of the objective is .SIGMA..sub.o
=.pi.(1.22.lambda./2.times.ON).sup.2 in which .lambda. is the
wavelength of the light beam and ON the numerical aperture of the
objective which is inversely proportional to the diameter of source
S. In practice, the true value .SIGMA..sub.1 of this surface is
well above .SIGMA..sub.0, due to aberrations of the lenses.
FIG. 7 shows variations in the signal-to-noise ratio S/B as a
function of the degree of real coherence .GAMMA..sub.1, i.e. the
ratio between the real diameter of image I and the diameter of the
pupil. The four curves .GAMMA..sub.1 to .GAMMA..sub.4 correspond
respectively to a circular source with .SIGMA..sub.1 =.SIGMA..sub.0
and .SIGMA..sub.1 =.SIGMA..sub.0 .times.10.sup.-3 and an annular
source with .SIGMA..sub.1 =.SIGMA..sub.0 and .SIGMA..sub.1
=.SIGMA..sub.0 .times.10.sup.-3. It can be seen that the noise
decreases rapidly when the source diameter increases. Taking
account of the characteristics of photosensitive resins with
.sigma..sub.1 of the order of 10.sup.-1 noise no longer has any
effect on the impression of the resin and the spatial coherence is
still very high, making it possible to retain the advantages in
connection with the resolution. Moreover, the high power of the
laser units permits very short exposure times.
The invention can be used not only for photorepetition, but also in
scale 1 projection systems and catadioptic projection systems.
* * * * *